This fantastic image is of a 'MoDi' placenta – monochorionic (one placenta) diamniotic (two amniotic sacs).

This occurs when blastocyst cells divide during days 4-8 after fertilization. This is late enough for the developing embryo to have created one placenta, but early enough that amniotic sacs aren’t present, so each twin will develop its own. They are almost always monozygotic, meaning that they come from one egg that is fertilized by one sperm, and therefore are identical.

Think of the two babies living in the same house (your belly), each having their own bedroom (amniotic bag), but sharing the plumbing (placenta) in the house. The risks associated with one placenta are related to the sharing the plumbing (risk for one to steal blood from the other), as well as the size of the bedrooms (how much of the placenta each kid got).

Virtually all monochorionic twin pregnancies share plumbing (placental blood connections). However, the majority (80-85%) have an even balance between the two babies, hence only 15-20% ever get twin-to-twin transfusion syndrome (TTTS) which is where one of the little ones starts stealing blood from his brother or sister, which can be of concern and needs to be monitored closely. 

During pregnancy, an examination of the placenta(s) and membranes via ultrasound can sometimes confirm chorionicity. An ultrasound scan, usually early in the pregnancy, will check for one or two placentas and the thickness of membranes. A thicker membrane can indicate di/di twins, while a thin membrane or the absence of a membrane can signal mo/di or mo/mo twins.

In Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin. 'MoDi'  twins occur in 0.3% of all pregnancies and 75% of all monozygotic twin pregnancies occur completely randomly.*

*https://www.fertilitysmarts.com/definition/1177/monochorionic-diamniotic-twins-mcda